As generally known, in wireless LAN, a wireless relay unit referred to as “access point (hereinafter, abbreviated as “AP”)” and multiple wireless terminals are connected via wireless communication. Since in this wireless LAN, those multiple wireless terminals and the AP (hereinafter, each of these elements will be simply referred to as a “device”) establish communication sharing a single space, a wireless signal can be transmitted from only one of those devices at a time. Therefore, it is necessary to adjust timing when transmission is performed among the devices.
DCF (Distributed Coordination Function) is disclosed as a timing adjustment method for wireless communication. As this timing adjustment method using the DCF, two examples are known as described in the following non-patent document. “Wireless LAN Medium Control (MAC) and Physical Layer (PHY) Specifications”, ANSI/IEEE Std 802.11, 1999 Edition.
The first method is as the following: After waiting a lapse of a predetermined time from the end of receiving a wireless signal from another device, wireless communication is started with a confirmation that there is no receipt of wireless signal from another device during the waiting period.
Specifically, as shown in FIG. 17, as a way of example, it is assumed that AP has transmitted DataP1 to the wireless terminal A (T10). In receipt of this DataP1, after a lapse of the shortest waiting period referred to as SIFS (Short Inter-Frame Space) (T20), the wireless terminal A gives a response of ACK signal addressed to AP (T21). When this communication of ACK signal is finished, each device waits for a period referred to as random backoff interval, so as to become a source of the next permitted transmission (T11, T22, and T32). This random backoff interval indicates a time period obtained by generating a random number (a value at least one) in the device itself, and adding the period corresponding to this random number, to a time period referred to as DIFS (Distributed Inter-Frame Space. Here it is assumed DIFS>SIFS). During this random backoff interval, each device continues to detect whether or not each device receives a wireless signal from another device.
If it is assumed that the random backoff interval of the wireless terminal A is the shortest, that is, if the wireless terminal A becomes a source of the permitted transmission, it sends the DataA1 signal to the AP (T23). In this case, since the random backoff intervals of the AP and the wireless terminal B are longer than the that of the wireless terminal A, the AP and the wireless terminal B do not send any data signal at the time when the wireless terminal A starts transmitting the DataA1 signal. As for the AP, since it receives the wireless signal from the wireless terminal A, in a state having not started wireless sending, the AP abandons to be a source of permitted transmission.
As thus described, the first method avoids a situation that multiple devices perform data transmission within the same time zone. However, in the case where the wireless terminal A fails to detect a transmission from the wireless terminal B, and the wireless terminal B fails to detect a transmission from the wireless terminal A, that is, in the case where the wireless terminals A and B have relations of hidden terminals with each other, the wireless terminal B cannot receive the DataA1 signal from the wireless terminal A. Therefore, at the end of own random backoff interval (T32), the wireless terminal B starts sending DataB1 signal to the AP (T33). Consequently, there occurs contention between the DataA1 signal from the wireless terminal A to the AP, and DataB1 signal from the wireless terminal B to the AP, thereby causing a situation that the AP is not capable of receiving a wireless signal normally. Therefore, the AP does not send an ACK signal either, which is to notify the wireless terminals of a normal signal receipt by the AP.
Since the wireless terminals A and B are not able to receive the ACK signal in response to each of the data signals sent out from the terminals respectively, the terminals sends again the DataA1 signal and DataB1 signal (T26, T36), after a lapse of new random backoff intervals (T25, T35). Even for the resent signals, since the terminals fail to detect mutually that wireless communication is established from each terminal, there is a possibility that the AP is not capable of receiving a signal normally due to contention again. It is to be noted here that the wireless terminal A receives the DataP1 signal from the AP, and transmits ACK signal (T21) after a lapse of SIFS (T20). Based on this ACK signal, the wireless terminals A, B, and AP start counting of the random backoff interval. However, if the wireless terminals A and B have relations of hidden terminals with each other, the wireless terminal B is not able to recognize the ACK signal (T21) sent from the wireless terminal A. Therefore, the wireless terminal B starts counting the random backoff interval (T32), assuming that an ACK signal was transmitted after a lapse of SIFS from the time when the DataP1 signal from the AP was received (T20).
The second method is as the following: Each wireless terminal sends RTS (Request To Send) record as a transmission request to the AP. Upon receipt of this RTS signal, the AP sends to any one of the terminals CTS (Clear To Send) signal which gives a clearance for transmission to only one of the wireless terminals having sent the RTS signal. Then, only the wireless terminal which has obtained the clearance starts data transmission.
Specifically, as shown in FIG. 18, for instance, it is assumed that the AP sends the DataP1 signal to the wireless terminal A (T10). Upon receipt of this DataP1, the wireless terminal A returns an ACK signal to the AP (T21) after a lapse of SIFS time (T20), as in the case of the first method described above. When the communication of this ACK signal is finished, each device waits for the lapse of random backoff interval (T11, T22, and T32).
If the random backoff interval of the wireless terminal A is the shortest, the wireless terminal A sends an RTS signal to the AP (T27). In this case, since the random backoff intervals of the AP and the wireless terminal B are longer than that of the wireless terminal A, the AP and the wireless terminal B do not send a signal at the time when the wireless terminal A starts transmitting the RTS signal.
The AP which received this RTS signal sends a CTS signal (T13) that gives the wireless terminal A, a clearance for transmission, after a lapse of SIFS time from this receipt of the RTS signal (T12). This CTS signal includes a time period necessary for the wireless terminal A to transmit data, specifically, information of duration (=SIFS+data transmission time from wireless terminal A+SIFS+ACK transmission time), after the CTS has been sent. Therefore, during this period, it is possible to suppress transmission from another wireless terminal.
Each wireless terminal which received the CTS signal determines whether or not this signal indicates a clearance for data transmission from its own terminal, and if the signal indicates a clearance for data transmission from own terminal, the wireless terminal sends a data signal to the AP after a lapse of SIFS from the receipt of the CTS signal. In the case here, since the CTS signal gives the clearance for transmission to the wireless terminal A, the wireless terminal A sends the DataA1 to the AP (T29) after a lapse of SIFS (T28) from the receipt of the CTS signal. In receipt of the DataA1 from the wireless terminal A, the AP returns an ACK signal (T15) to the wireless terminal A, after a lapse of SIFS (T14).
If the wireless terminals A and B have relations of hidden terminals from each other, and the wireless terminal B fails to detect that the wireless terminal A has sent the RTS signal (T27) to the AP after a lapse of its random backoff interval from the time when the wireless terminal A returns the ACK signal (T21) to the AP, the wireless terminal B, whose random backoff interval is shorter than the wireless terminal A next in order, tries to send an RTS signal (T34) subsequent to the RTS signal (T27) sent from the wireless terminal A. However, as described above, the AP sends a CTS signal giving the clearance for transmission to the wireless terminal A (T13). Therefore, the wireless terminal B stops sending of this RTS signal.
After the wireless terminal A sends the DataA1 (T29) and the AP returns an ACK signal to the wireless terminal A (T14, T15), each device again waits for a lapse of new random backoff interval (T36). If the random backoff interval of the wireless terminal B is the shortest, the wireless terminal B sends an RTS signal to the AP, prior to sending from another wireless terminal (T37). Therefore, in response to this RTS signal, the AP sends a CTS signal giving a clearance for transmission to the wireless terminal B. In receipt of this CTS signal, the wireless terminal B sends a data signal to the AP.